1. Field of the Invention
The present invention relates to a self-oscillation type switching power supply apparatus.
2. Description of the Related Art
As a self-oscillation type switching power supply apparatus, ringing choke converters have been conventionally used in most cases. FIG. 6 is a circuit diagram showing a conventional ringing choke converter. As seen in FIG. 6, a switching transistor Q1 is connected in series with a primary winding N1 of a transformer T. A controlling circuit including a phototransistor PT as a photodetecting element of a photocoupler is connected to a feedback winding N.sub.B of the transformer T. Further, a controlling transistor Q2 is connected across the gate--source of the switching transistor Q1.
A rectifying smoothing circuit comprising a rectifying diode D3 and a smoothing capacitor is connected across a secondary winding N2 of the transformer T. For the output of the rectifying smoothing circuit, a resistor voltage-dividing circuit comprising resistors R9, R10, and a voltage detecting circuit comprising a shunt regulator SR, a light emitting diode PD of the above-mentioned photocoupler, and a resistor R8 are provided.
The operation of the circuit shown in FIG. 6 will be now described. At start up when a power supply is put to work, voltage is applied to the gate of the switching transistor Q1 through a starting resistor R1, so that the switching transistor Q1 is turned on. Thereby, an input power supply voltage is applied to the primary winding N1 of the transformer T, so that a voltage is produced in the feedback winding N.sub.B in the same direction as that in the primary winding N1. The voltage signal is provided as a positive feedback signal to the gate of the switching transistor Q1 through a resistor R2 and a capacitor C2. The voltage produced in the feedback winding N.sub.B also causes a charging current to flow to a capacitor C3 through a diode D1, resistors R3, R5. and a photocoupler PT. When the charging voltage of the capacitor C3 exceeds a base--emitter forward voltage of the controlling transistor Q2, the controlling transistor Q2 is turned on. Thereby, the gate--source voltage of the switching transistor Q1 becomes substantially zero, so that the switching transistor Q1 is forced to turn off. At this time, a voltage is produced in the secondary winding of the transformer T in the forward direction with respect to the rectifying diode D3, and energy stored in the transformer T while Q1 is on is released through the secondary winding N2. At this time, the capacitor C3 is reverse-charged with the flyback voltage of the feedback winding N.sub.B supplied through resistors R6, R7 and a diode D2.
When the voltage of the capacitor C3 gets to be lower than the base--emitter forward voltage of the controlling transistor Q2, the controlling transistor Q2 is turned off. The energy stored in the transformer T is released from the secondary, and the current in the rectifying diode D3 becomes zero. Then, the respective winding voltages of the transformer T become zero. The circuit is returned to its initial state, and then, the switching transistor Q1 is turned on. The above-described operation is then repeated periodically.
The output voltage on the load side is detected as a voltage divided by the resistors R9, R10. The divided voltage is applied as a detection voltage and a control voltage for the shunt regulator SR. The shunt regulator SR regulates the conducting quantity of electricity for the light emitting diode PD of the photocoupler in dependence on the detection voltage. By the regulation, the quantity of light accepted by the phototransistor PT, as a photodetector of the photocoupler, is changed, causing the impedance of the phototransistor PT to change. As a result, the charging time constant of the capacitor C3 is changed. As the output voltage is lower, so the charging time constant is higher. That is, as the output voltage is lower, the period from the time when the switching transistor Q1 is turned on until it is forced to be turned off by the controlling transistor Q2, namely, the "on"--state time period of the switching transistor Q1 is longer. This acts so that the output voltage is increased. Thus, a constant voltage control for keeping the output voltage constant is achieved.
In the conventional self-oscillation type switching power supply apparatus as shown in FIG. 6, when the output current Iois increased and the output voltage Vo is decreased, the impedance of the phototransistor PT is increased by the feedback control carried out through the photocoupler. However, in the event that the output power Io goes into an overcurrent state, and the impedance of the phototransistor PT reaches a maximum, the "on"--state time period of the switching transistor Q1 can not be further lengthened. As a result, the output voltage Vo begins to drop. FIG. 7 is a graph showing the relation between the output current and the output voltage. With droping of the output voltage Vo, the flyback voltage of the feedback winding N.sub.B is reduced, and the reverse-charge voltage of the capacitor C3 is reduced, so that the base--emitter voltage Vbe of Q2 is directed toward the negative with difficulty. That is, Q2 is turned on in a short time, so that the "on"--state time period of Q1 is reduced to a minimum.
In the event that the load is short-circuited, and the output voltage Vo gets to be substantially zero, the controlling transistor Q2 is turned on and off, repeatedly, and the switching transistor Q1 is turned on and off repeatedly, with short "on"--state time periods. As a result, a constant short circuit current flows. There is a danger that the short-circuit current causes not only a useless power loss but also abnormal heating of and damage to the switching transistor Q1, the rectifying diode D3, and the load.